1. Field of the Invention
The present invention relates to a solid electrolytic capacitor used in a variety of electronic devices.
2. Background Art
With the recent digitization of electronic devices, solid electrolytic capacitors used in these devices are increasingly demanded to reduce equivalent series resistance (hereinafter, referred to as ESR) in a high frequency range. The following is a description of the structure of a conventional solid electrolytic capacitor with reference to FIGS. 18 and 19. FIG. 18 is a sectional view showing the structure of the conventional solid electrolytic capacitor, and FIG. 19 is a sectional view showing the structure of a capacitor element of the solid electrolytic capacitor.
As shown in FIG. 19, capacitor element 69 includes anode body 61, insulating layer 63, dielectric oxide film 64, solid electrolyte layer 65 and cathode layer 68. Anode body 61, which is planar, is made of a valve metal such as tantalum, aluminum, niobium and titanium. Insulating layer 63 divides anode body 61 into anode exposed portion 62 and the remaining portion. Dielectric oxide film 64 is formed on the surface of the remaining portion of anode body 61, which is divided from anode exposed portion 62. Solid electrolyte layer 65, which is made of a conductive polymer, is formed on the surface of dielectric oxide film 64. Cathode layer 68 is composed of carbon layer 66 formed on the surface of solid electrolyte layer 65, and silver paste layer 67 formed on the surface of carbon layer 66. These layers are sequentially formed to compose capacitor element 69.
As shown in FIG. 18, the solid electrolytic capacitor is composed of a plurality of laminated capacitor elements 69. Anode exposed portions 62 of capacitor elements 69 are welded together to form anode portion 70. Cathode layers 68 of capacitor elements 69 are bonded together via conductive adhesive layers (hereinafter, referred to as adhesive layers) 71 to form cathode portion 72.
Adhesive layer 71 is formed from conductive adhesive paste which is a mixture of silver powder, an organic binder and an organic solvent. More specifically, first, the conductive adhesive paste is applied quantitatively on the lamination planes of cathode layers 68. Then, capacitor elements 69 are laminated together, pressed to spread the conductive adhesive paste between cathode layers 68, and heated to harden the conductive adhesive.
Anode terminal 73 is connected to anode portion 70 by resistance welding, and cathode terminal 75 is connected to cathode portion 72 using conductive paste 74. Packaging resin layer 78 entirely coats laminated capacitor elements 69. This is how the solid electrolytic capacitor is structured. A solid electrolytic capacitor of this type is disclosed in Japanese Patent Unexamined Publication No. H03-145115.
In this conventional solid electrolytic capacitor, however, the heating and hardening of the conductive adhesive for adhesive layers 71 causes the organic solvent to vaporize and generates cavities 76 in the interface between cathode layers 68 and adhesive layers 71. As another problem, if the conductive adhesive is applied insufficiently, when it is spread between cathode layers 68, there may be caused non-formation portions 77 of adhesive layers 71 between cathode layers 68. Thus, it is difficult to spread the conductive adhesive paste between cathode layers 68 to form adhesive layers 71 with high precision. Cavities 76 or non-formation portions 77 cause a reduction in the bonded area between cathode layers 68 and adhesive layers 71. As a result, ESR is increased.
On the other hand, if the conductive adhesive is applied too much, the adhesive may be pushed out from cathode layers 68 or creep up to cause adhesive layers 71 to reach as far as insulating layers 63 or even as far as anode exposed portions 62. This leads to an increase in leakage current and may cause a short circuit. Such situations can be avoided by applying less amount of the conductive adhesive to reduce the bonded area between cathode layers 68 and adhesive layers 71. By doing so, the conductive adhesive is never pushed out to form protrusions on the side faces of cathode layers 68. This can prevent the outside moisture from entering capacitor elements 69 via the conductive adhesive to increase leakage current. However, as mentioned above, applying a reduced amount of the conductive adhesive reverses a reduction in ESR.
In particular, when a plurality of capacitor elements 69 are laminated together in which cathode layers 68 have a plurality of gaps formed therebetween, the conductive adhesive paste is spread differently in each gap. This makes it further difficult to form adhesive layers 71 with high precision. As a result, ESR is increased.